The AggregateCommandRequest object is parsed on the top level of the request, but that request may not necessarily be available to all the document source commands on the pipeline.  These document source commands may not contain the same context as the top-level aggregate request does, which means we should build a common serialization context used by all document source commands in order to ensure all serialization/deserialization actions are consistent with the structure of the aggregate request.  Because these commands share an Expression Context, we can place a SerializationContext object there to make it accessible to any document source down the line.

As the goal of these changes are in pursuit of fixing tests that will fail once serializeForCommands and deserializeForCommands are plumbed in, no new tests are being added for this ticket.